Barium containing silicate glasses exhibit excellent strength and a high refractive index as well as particularly desirable density, refractive index, and stability for dental composites. In a typical dental procedure, glass powder (usually .ltoreq.1 .mu.m and appropriate refractive index) and liquid monomer are mixed to prepare a thick paste. This paste is then molded into a desired shape, and the monomer exposed to ultraviolet or visible radiation. The result is a durable dental surface with an optical appearance that corresponds with the surrounding tooth surfaces.
One early method for preparing barium-containing silicate powders is by grinding a bulk mixture of oxides and carbonates. Unfortunately, the resulting powder is irregular in shape with a broad particle size distribution. The grinding process also can lead to undesirable contamination. The art looked for ways to produce spherical glass powders economically and in quantity.
Spherical powder is much preferred over irregular particles. The first is flow: the uncured pastes made with spherical particles flow better than those made using irregular particles of same weight fraction glass. As a result, the solids content of the mixture may be increased while retaining comparable flow properties.
Pyrolysis of an atomized liquid spray of precursor solution was one useful method for producing generally spherical particles. In such a process, a precursor solution containing the elements desired in the final glass is atomized to produce an aerosol (or a mist). The aerosol particles are then dried by evaporation of the solvent and heated to a temperature sufficiently high to convert the precursor compounds to the product glass particles. The particle size distribution of the resulting powder was controlled by a combination of aerosol size distribution, the solute content and by mechanical separators. Glass powders of regular, generally spherical shape could be produced with particle sizes in the micron and submicron range. The need for further grinding was eliminated with the corresponding source of potential contamination.
Spray pyrolysis processes require a stable source of liquid precursor solution. In addition, the cost of the precursor solution must be within certain limits for economic production, and the precursor should be reasonably safe to handle. These competing requirements have posed certain obstacles for the production of barium-containing silicate glass powders by spray pyrolysis and for barium aluminoborosilicate (BABS) glass powders in particular.
Previous attempts to produce a stable precursor of BABS solution have included various combinations of acetates, nitrate, or chlorides. Such solutions were hampered by precipitation of one or more components leading to cloudy particles that exhibited a gritty internal structure which was unsuitable for use in dental composites. Later attempts required use of silicon alkoxide and barium perchlorate. Unfortunately, silicon alkoxide is an expensive source of silica and is flammable. The barium perchlorate is also expensive and not readily available.
It would be desirable to have a precursor solution for BABS powders that was mote cost effective, nonflammable, and more commercially available than the previous silicon alkoxide/barium perchlorate system.
It is desirable for the preparation of BABS powders that the particle size of the final powder be in the range of 0.1-2 .mu.m. This final size range dictates that the droplet size be within the range of 0.3-9 .mu.m, i.e., 3-4 times the final glass particle size. Conventional atomization techniques for producing droplets in such a size range require the use of pneumatically assisted nozzles.
But not all pneumatically assisted nozzles are commercially practical. Pneumatic nebulizers that are often used in respiratory therapy require large volumes of carrier gas and require an aerodynamic separator such as an impactor or a cyclone to remove droplets larger than 8 .mu.m. A more advantageous ratio of droplet volume to carrier gas volume is needed to minimize energy costs associated with heating large volumes of carrier gas.
Other options for producing small droplets include high frequency (about 1-3 MHz) ultrasonic nebulizers and very high pressure jets. No gas is used in atomization, but only as a carrier to transport the droplets to the reactor. Ultrasonic and other such acoustic nebulizers are commercially available that can reach droplet concentrations as high as 10.sup.7 particles/cm.sup.3.
Another method is discussed in Aksay et al (U.S. Pat. No. 5,061,682). This patent describes a method to produce mixed oxide particles in the micrometer size range. Conventional spray nozzles are used to produce the spray containing the precursor compounds in solution with an ignitable compound. The combustion of the ignitable compound shatters the larger (&gt;10 mm) solution droplets into smaller particles. Unfortunately, this method poses a real potential to contaminate the product with combustion byproducts and lacks direct control over the resulting particle size.